When the power exceeding an allowed input is continuously supplied to a loudspeaker device or to a loudspeaker unit, the temperature of the voice coil of the loudspeaker unit is excessively increased, as a result of which the voice coil skin is gradually carbonized to cause partial shorting. In this partially shorted state, an excessive current, for example, a current of several to tens of amperes, flows through the voice coil, with the consequence that the remaining normal portions of the voice coil become red-hot to cause fuming, shorting, abnormal heat evolution or line breakage. In case of excessive heat evolution, ignition may sometimes be produced. In particular, if a power amplifier of a large output is frequently used, abnormal heat evolution is increased, thus producing an extremely risky situation.
With this in mind, such a loudspeaker device 1, provided with a circuit for protecting a loudspeaker unit from an excess input state, as shown in FIG. 1, has been proposed. This loudspeaker device includes a loudspeaker unit 2 as a subject of protection against any excessive input state. The loudspeaker unit 2 is mounted to a cabinet 3. In the illustrated example of the cabinet 3, a port 4 for reflex is provided on its front surface.
The back surface of the cabinet 3 carries input terminals 5A, 5B. To one 5A of these terminals is connected the loudspeaker unit 2 via a lamp 6, such as a small-sized incandescent lamp, whereas, to the other input terminal 5B, there is directly connected the loudspeaker unit 2. A power amplifier 7 is connected to the input terminals 5A, 5B. An audio output of the power amplifier 7 is supplied via a lamp 6 to the loudspeaker unit 2. If, in this case, the output of the power amplifier 7 is only small, the signal current flowing through the lamp 6 is also small, so that the lamp 6 is not lit, with its resistance being small. Thus, the audio output of the power amplifier 7 is supplied to the loudspeaker unit 2, substantially unchanged, so that a desired acoustic output is produced.
However, if the output of the power amplifier 7 continues to be of a large value for an extended time interval, a large current continues to flow through the lamp 6, so that the lamp 6 is lit, with the resistance of the lamp being then increased. Consequently, the signal power supplied to the loudspeaker unit 2 is limited, so that the loudspeaker 2 may be protected against excess input.
Thus, the loudspeaker device 1, shown in FIG. 1, is able to protect the loudspeaker unit 2 from excess input by the lamp 6.
In the case of the loudspeaker device 1, described above, the lamp 6 is lit under a condition in which the input of an excess level is continuously supplied, with the result that the inside of the cabinet 3 becomes illuminated due to light emitted by the lamp 6, or the light may leak through a port 4, so that the user may feel disagreeable. In particular, if the loudspeaker device 1 is used in conjunction with video equipment, such light leakage is objectionable for image appreciation.
Moreover, in the above-described loudspeaker device 1, it may be an occurrence that, if part or all of the voice coil of the loudspeaker unit 2 becomes shorted due to excess input, so that its impedance is lowered, the loudspeaker unit 2 or the lamp 6, as load, does not appear to be shorted, when looking from the power amplifier 7, such that the power amplifier 7 continues to be in operation as normally. If such state persists, the output of the power amplifier 7 continuously flows into the lamp 6, with the risk that the temperature of the lamp 6 is increased excessively.